The present invention relates to a new fixing agent and a process for improving the fastness properties, especially to wet-fastness properties such as the fastness to washing, to hot water, to perspiration or to water, of dyeings on polyamide fibers.
Generally, polyamide fibers are dyed by acid-dyestuffs. However, the employment of only acid-dyestuffs merely gives very poor fastness properties of dyeings. For this reason, there have been adopted a variety of means for improving the fastness properties of dyeings by employing fixing agents. Among them, a two bath method employing tannic acid and tartar emetic has been known to be the most effective in the viewpoint of fixing dyestuffs on the fibers. This two bath method gives fairly good wet-fastness properties but is poor in the operating efficiency because of its procedure consisting of two steps, i.e. tannic acid treatment and tartar emetic treatment, which results in a high cost. Further, according to this method, the color matching of dyed fibers is very difficult because the colors of dyeings on fibers are liable to change of shade. The employment of tartar emetic necessitates an additional facilities for removing toxic tartar emetic from the waste water of bath. Besides, the supply of tannic acid is unstable from the viewpoint of its raw material and its cost is fairly high.
As one of alternative means for improving the above-mentioned defects of such tannic acid-tartar emetic method, there has been proposed a method employing as a fixing agent a condensation product of sulfonated 4,4'-dihydroxydiphenylsulfone and a lower aliphatic aldehyde such as formaldehyde or acetaldehyde, for instance, as described in British Pat. No. 975,307. That is to say, in this patent, there is described a method for improving the wet-fastness properties of dyeings on polyamide fibers by after-treating a dyed fiber with the condensation product at 80.degree. C. However, even this method is still inferior to the tannic acid-tartar emetic treatment with respect to the effect of fixing dyestuffs on fibers, especially is poor in resistance against steam setting.